2016's Final Fantasy XV will be getting new content until next year. This will be in the form of new episodes focussing on the game's supporting cast.
Speaking to 4Gamer (via Gematsu) Final Fantasy XV's director, Hajime Tabata, Final Fantasy XV has an "average player satisfaction rate" of 62.4 percent for the game's scenario in totality. Due to this Tabata believes that it was necessary to "ensure a suitable development period" for its new content. Therefore not all of it will be out in 2018.
What's more is, content over and above the current planned episodes that take place in the Final Fantasy XV universe, but not core to the main plot is being considered. Reason being Tabata said in an interview with Famitsu that Final Fantasy XV's existing players aren’t interesting in the team filling “in the gaps of the main story anymore.”
That being said, the next Final Fantasy XV episode focusses on Ardyn - the main villain of Final Fantasy XV. Final Fantasy XV: Episode Ardyn was announced in November last year. An episode featuring Final Fantasy XV hero Noctis' love interest Luna is also a possibility.
All of this puts Final Fantasy XV in an interesting place. These announcements coupled with the existing free updates and patches to Final Fantasy XV have ensured that those who bought and played the game on day one (or earlier thanks to a street date break) haven't got anything close to the experience Square Enix intended to deliver to its fans. Throw in the recently announced Final Fantasy XV Royal Edition - which most likely is the base game minus all its patches and additional content - which you're expected to download (in excess of 50GB), and you have a game that's far from accessible.
This makes Final Fantasy XV a far cry from what was promised prior to launch. Then again, considering the debacle that was its development and we're surprised that it was cancelled altogether.